Carbon Frame / Alloy Seat Post

James T
James T Posts: 104
edited June 2013 in Workshop
I struggled to remove the alloy seat post from my Planet X carbon frame the other day after not moving it for a few years probably. There was a strange white substance on the seat post and inside the frame. I do not think this was corrosion at it wiped clean with a damp rag and the black finish on the seat post was still intact - was it the dried remnants of some form of assessmbly paste or was it a form of corrosion?

Should I use some form of assembly paste to prevent the post from seizing again, and aid friction to prevent slippage (if both facets are possible and not counter intuitive), and if so what do people recommend for alloy seat posts in carbon frames.

Kind regards

James

Comments

  • chrisvanderkaap
    chrisvanderkaap Posts: 323
    it's called ceramic assembly paste. it's pretty good for just about anything with metal treads on your bike because it stops stuff from seizing up. Shimano therefor also calls it anti-seize. With carbon on carbon interface (and in this case carbon/aluminum) especially concerning seat posts i'd rather use carbon friction paste which replaces the ceramic balls with pastic ones to introduce friction and reduce slippage, it's bonding agent is also cleaner than regular anti-seize. I'd recommend that for refitting your post.

    either way, don't worry. It's good you took the effort to reassemble some fitted components as it's good to check if they're properly fitted and lubed.

    stay in good shape!

    best,

    C
  • mmacavity
    mmacavity Posts: 781
    http://www.boeing.com/commercial/aeroma ... rrosn.html

    "For example, graphite fibers, which are used to reinforce some plastic structure, present a particularly challenging galvanic corrosion combination. The fibers are good electrical conductors and they produce a large galvanic potential with the aluminum alloys used in airplane structure."
  • sungod
    sungod Posts: 16,607
    ^^^this

    the white stuff was likely to be corrosion, it causes posts to jam because the corrosion products take up more space than the original metal

    after cleaning out the seat tube and post apply assembly paste over the full length to be inserted in the seat tube

    if the post is tight, the tacx assembly paste is a good choice, if the post is a bit looser the grittier finish line paste may be better

    either way, if you do many wet rides it's worth removing the post every year to clean out gunk and re-apply paste
    my bike - faster than god's and twice as shiny
  • Monty Dog
    Monty Dog Posts: 20,614
    Could be galvanic corrosion, particularly if bike has been ridden in the wet. The water acts as a conductor between the carbon and bare alloy, resulting in a precipitation of aluminium oxide on the mating surfaces. The white powdery residue can jam the post in place, or worst-case, split the carbon. Apply an anti-seize/barrier compound and regularly check/reapply. Probably one of the most common causes of a trashed frame after crashing.
    Make mine an Italian, with Campagnolo on the side..

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